1. Field of the Invention
This invention relates to a semiconductor laser device capable of being used for optical fiber communication which needs ultra high speed optical modulation, or for optical logic circuits, optical connection, etc., utilized in optical computers.
2. Description of the Related Art
Hitherto, a method of modulating the intensity of output power from a quantum well by applying an electric field to a quantum well structure in the lamination direction of the structure to vary the quantum states has been practiced, and discussed in Oyo Butsuri [Journal of Applied Physics (Japan)], Vol. 55, p. 210, 1986. The principle of modulation used in the method will be explained below while referring to FIGS. 2A and 2B. Referring to FIG. 2A, there are shown the quantum level (broken line) and the wave functions of electrons and holes, under zero applied electric field. Both the electrons and holes are localized in the quantum well layer, and the wave functions thereof are each symmetrical with respect to the center of the well. When an electric field is applied in the lamination direction of the quantum well structure, on the other hand, the energy bands are inclined as shown in FIG. 2B, so that the symmetry of the wave functions is lost. That is, the wave function of electrons is localized to the left side where the energy is lower, whereas the wave function of holes is localized to the right side, as shown in the figure. In other words, the position of electrons and the position of holes differ from each other on a spatial basis, as shown in FIG. 2B.
In this specification, the difference between the peak position of the wave function of electrons and the peak position of the wave function of holes is defined as the "distance between the position of electrons and the position of holes", as shown in FIG. 2B. Thus, when an electric field is applied externally in the lamination direction of the quantum well structure as mentioned above, the wave function of electrons and the wave function of holes are spatially separated from each other in the quantum well. That is, the "distance between the position of electrons and the position of holes" is increased. As a result, the spatial overlap of the wave function of electrons and the wave function of holes is decreased, resulting in a decrease in the probability of optical transition, namely, in the oscillation strength. In short, the intensity of output power is varied depending on whether an electric field is present or not. The speed of modulation, or the rate of variation in the intensity of output power depending on the presence or absence of an electric field, is extremely high--on the order of picoseconds. Thus, it is possible to increase the modulation speed by one or two orders of magnitude, as compared with the speed attained in a direct modulation system of semiconductor laser according to the prior art.